There are two general ways by which cells die. An easily recognized pathway is necrosis, a process of cell death usually resulting from severe and sudden injury. In necrosis, changes in cellular homeostasis occur with loss of membrane integrity. Dysregulation of osmotic pressure results and, as a consequence, the cells swell and finally rupture. The cellular contents are then spilled into the surrounding tissue space and, usually, an inflammation response ensues. A second form of cell death is apoptosis. This cell "suicide" pathway or programmed cell death often occurs so rapidly that in some biological systems the apoptotic process is difficult to ascertain. Indeed, it has been only in the past few years that the involvement of apoptosis in a wide spectrum of biological processes has become recognized. Apoptosis is a fundamental physiological pathway of cell death, highly conserved throughout evolution, and playing a major role in development, viral pathogenesis, cancer, autoimmune diseases and neurodegenerative disorders.
Inappropriate increases in apoptosis may cause or contribute to a variety of diseases, including AIDS, neurodegenerative diseases (e.g. Alzheimer's Disease, Parkinson's Disease, Amyotrophic Lateral Sclerosis (ALS), retinitis pigmentosa and other diseases of the retina, myelodysplastic syndrome (e.g., aplastic anemia), toxin-induced liver disease (e.g., alcoholism) and ischemic injury (e.g., myocardial infarction, stroke, and reperfusion injury). In addition, disruption of normally occurring apoptosis has been implicated in the development of some cancers (e.g. follicular lymphoma, p53 carcinomas, and hormone dependent tumors), autoimmune disorders (e.g., lupus erythematosis and multiple sclerosis) and viral infections (e.g., herpes virus, poxvirus, and adenovirus infections).
Mature CD4.sup.+ T-lymphocytes in patients with HIV-1 have been observed to respond to stimulation with mitogens or super-antigens by undergoing increased apoptosis. The great majority of these cells are not infected and similar inappropriate antigen-induced apoptosis could be very important in the destruction of this vital part of the immune system early in HIV infection.
Baculoviruses encode inhibitors of apoptosis proteins (IAPs). These proteins inhibit the apoptosis which otherwise occurs when insect cells are infected by the virus. Baculovirus IAP proteins work in a manner which is thought to be independent of other viral proteins. The baculovirus IAP genes include sequences encoding a ring zinc finger-like motif which is presumed to be involved in the direct binding of DNA.